In optical communication networks, multi-channel support using a Dense Wavelength Division Multiplexing (DWDM) technology which enables high-capacity connections of Point-to-Point (P2P) and an Optical Cross Connect (OXC) using a frequency selection switch (Wavelength Select Switch (WSS)) having a multi (three or more) direction or degree switching function have been introduced, for the speeding-up of a network which will be accompanied by increase in communication amount and diversification of services in the future. The introduction of the OXC enables networks to be optimally reconstructed according to use states of the networks.
Further, as described in Non-Patent Document 1, a transceiver aggregator, in which a plurality of optical transceivers, a WSS, and the like are installed, is installed between an optical communication network and a client device, so the sharing of optical transceivers is in progress. Thus, the number of the installed optical transceivers can be reduced and Capital Expenditure (CAPEX) can be decreased. A method of sharing the optical transceivers has a variety of forms including a method in which each fiber direction or degree shares the optical transceivers and a method in which a plurality of fiber directions or degrees in bulk share the optical transceivers.
In addition, network update automation is in progress by the optical transceivers and the WSS within the network apparatus being controlled by a computer. An example of a network management system is disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2010-161501). The network management system of Patent Document 1 dynamically changes the configuration of a network depending on the use state in order to reduce power consumption of the entire network.
Prior to the automation, workers are required to go to each node station and to change network apparatuses at the time of update. However, due to the automation of the network update, the workers are not required to change the apparatuses, and Operating Expense (OPEX) also can be decreased. In addition, as described in Patent Document 1, the network can be dynamically optimized depending on the use state of the network.
Due to development of the network technology, it is expected that a plurality of unused optical transceivers are installed in each network apparatus and are dynamically set to an in-use state or a not in-use state depending on the use state of the network.
The unused optical transceivers are on standby in a state where the optical output is zero so that the light does not leak to the normal network. At this time, backup optical transceivers are on standby in a state where the backup optical transceivers are shared among the fiber directions or degrees of one or more and are activated to be connected to a network, when a new optical path is connected or a failure is recovered. It is desirable that the optical path setting time may be serviced instantly. Particularly, rapid optical path setting is required for restoration (network switching) at the time of failure recovery to minimize the impact of failures.
Optical transmission and reception devices are present as analog devices in the optical transceivers. With the introduction of DWDM technology in recent years, devices such as frequency tunable lasers and frequency tunable filters capable of controlling frequencies are used as the optical transmission and reception devices.
Although various types of frequency tunable lasers and filters exist, tuning parameters such as a refractive index, an electric voltage, an electric current or a temperature of a medium such as a semiconductor, a dielectric, a liquid crystal, or the like, are controlled in an analog manner at high precision for setting the frequency of these devices. Since these frequency control delay mechanisms have no significant difference therebetween, they will be described using a frequency tunable laser described in Patent Document 2 (Japanese Patent No. 4596181).
Patent Document 2 is a patent relating to a frequency tunable laser of an external resonator type. The frequency tunable laser described in Patent Document 2 has a configuration in which an external resonator is disposed between a semiconductor optical amplifier and an external mirror and at least a frequency selection filter and a frequency tunable filter are disposed in the inside thereof. The frequency selection filter is a periodic transparent filter with a transparent peak only in channels defined for use in communication applications, and a solid etalon filter is generally used therefor.
As illustrated in FIG. 2 of Non-Patent Document 2, one frequency is selected by a frequency tunable filter from a plurality of peak frequencies of the frequency selection filter, and laser is oscillated in any frequency. In other words, the number of frequency tuning parameters of the frequency tunable laser described in Patent Document 2 is one and the peak frequency of the frequency tunable filter is tuned, thereby allowing the frequency variable characteristic to be achieved. The frequency tunable filter described in Patent Document 2 uses a liquid crystal-based frequency tunable mirror, and the frequency tuning parameter is an alternating voltage value to be applied to the liquid crystal. The refractive index of the liquid crystal is changed by changing the alternating voltage value so as to control the frequency, and thus the greater the voltage changes, the greater the frequency changes.
Channel setting in the frequency tunable laser described in Patent Document 2 is as follows. The alternating voltage value of the frequency tunable filter is first changed, and a coarse adjustment is performed on the peak frequency. Moreover, a fine adjustment combined with the stable operation of the laser beam is performed in the frequency tunable laser. This fine adjustment includes a constant light output control by a dither control, a frequency control for matching an oscillation frequency to an International Telecommunication Union (ITU) channel with high precision, and a phase control for operating a laser light at low noise.
In addition, as described in Non-Patent Document 5, various types such as a type which has two frequency tuning parameters exist as frequency tunable filters.
Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2001-249053) discloses a frequency tunable laser used as a local oscillator for frequency selection.